This invention relates to the addressing of bistable nematic liquid crystal devices.
One known bistable nematic liquid crystal device is described in WO-97/14990, PCT/GB96/02463, GB98/02806.1, EP96932739.4 and has been described a zenithal bistable device (ZBD(trademark)). This device comprises a thin layer of a nematic or long pitch cholesteric liquid crystal material contained between cell walls. Optically transparent row and column electrode structures arranged in an x,y matrix of addressable pixel allow an electric field to be applied across the layer at each pixel causing a switching of the material. One or both cell walls are surface treated to permit nematic liquid crystal molecules to adopt either of two pretilt angles in the same azimuthal plane at each surface. Opposite surfaces may have pretilt in differing azimuthal planes. The two states are observed as a dark (e.g. black) and a bright (e.g. light grey) state. The cell can be electrically switched between these two states to allow information display which can persist after the removal of power; i.e. the liquid crystal material is latched into either of the two allowed states and remain in the one latched state until electrically switched to the other latched state.
Another bistable nematic device is described in WO99/34251, PCT/GB98/03787. This uses grating structures to provide bistable alignment similar to WO-97/14990 but uses a negative dielectric anisotropy material.
The terms switching and latching need some explanation: in monostable nematic devices, the effect of a suitable applied electric field is to move the liquid crystal molecules (more correctly the director) from one alignment condition to another, i.e. from a zero applied voltage OFF state to an applied voltage ON state. In a bistable device, the application of a voltage may cause some movement of the liquid crystal molecules without sufficient movement to cause them to permanently move into a different (one of two) state. In the present application, the term switch and latch are used to mean the molecules are caused to move from one bistable state to the other bistable state; where they remain until switched or latched back to the first state.
The term same azimuthal plane is explained as follows; let the walls of a cell lie in the x,y plane, which means the normal to the cell walls is the z-axis. Two pretilt angles in the same azimuthal plane means two different molecular positions in the same x,z plane.
Another bistable nematic liquid crystal device is described in GB-2,286,467. This uses a grating alignment surface to give two stable states in two different azimuthal planes.
Most presently available liquid crystal devices are monostable and are addressed using rms. addressing methods. For example twisted nematic and phase change type of liquid crystal devices are switched to an ON state by application of a suitable voltage, and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level. In these devices the liquid crystal material responds to the rms. value of the electric field. Various well-known addressing schemes are used; all use ac rms. voltage values. This is convenient because liquid crystal material deteriorat if the applied voltage is dc.
EP 569,029 describes a long pitch cholesteric liquid crystal display having two metastable switched states. The material is first switched to a Frederick""s transition, then switched with other voltages to either of the two metastable states. Each state lasts for about 10 seconds after voltage is removed; i.e. the display has (temporary) bistability providing the display is continually addressed.
Another type of device is the ferroelectric liquid crystal display (FELCD) which can be made into bistable device with the use of smectic liquid crystal materials and suitable cell wall surface alignment treatment. Such a device is a surface stabilised ferroelectric liquid crystal device (SSFELCDs) as described by: -L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). Then device switch upon receipt of a suitable unipolar (dc) pulse of suitable voltage amplitude and time. For example a positive pulse switches to an ON state, and a negative pulse switches to an OFF state. A disadvantage of this in that the material will degenerate under dc. voltages. Therefore the many known addressing schemes must ensure a net zero value dc. For example by periodically inverting all voltages.
Known addressing schemes for bistable smectic devices include those described in EP-0,542,804 PCT/GB91/01263, EP-0,308,203, EP-0,197,742, Surgey et al ferroelectric 1991, vol. 122 pp63-79 etc. Some use mono pulse strobe pulses, others bipolar strobe pulses in combination with bipolar data pulses.
Bistable nematic devices, as mentioned above, switch between or latch into their two bistable states upon receipt of suitable unipolar (dc) pulses. This may allow use of existing addressing schemes previously used for ferro electric bistable devices. However, the switching characteristics of bistable nematic devices are different from that of ferro electric bistable devices.
The present invention addresses the problem of switching bistable nematic liquid crystal devices by providing new addressing schemes, which take account of the different switching characteristics of bistable nematic devices.
According to this invention a method of addressing a bistable nematic device formed by two cell walls enclosing a layer of nematic or long pitch cholesteric liquid crystal material with electrode structures carried by the walls to form a series of row electrodes on one wall and a series of column electrodes on the other wall to form a matrix of intersecting regions or pixels with a wall surface treatment on at least one wall providing a molecular alignment permitting the molecules at or adjacent the wall to align into two different stable states upon application of appropriate unipolar voltage pulses, the method comprising the steps of:
applying a row waveform to each row in a sequence whilst simultaneously applying one of two data waveforms to each column electrode whereby each pixel can be independently switched between two bistable states;
the row waveform having a period of at least two time slots, at least two unipolar pulses for switching the device to a first state, and at least two unipolar pulses for switching the device to a second state,
both data waveforms having a period of at least two time slots with a unipolar pulse in each time slot, with at least one data waveform shaped to combine with the row waveform to cause a switching to one latched state
whereby each pixel can be addressed to latch into either stable state to collectively provide a desired display, with a substantially net zero dc voltage applied to the device.
Preferably the alignment treatment on a cell wall is arranged to give two different switching characteristics; namely lower voltage/time values for switching from one latched state to the other latched state. This may be arranged by variation of the height of grooves in a grating structure, and/or variation of the period of the grating, and/or selection of a surfactant on the grating, and/or selection of material elastic constants. The surfactant may be lecithin or a chrome complex surfactant.
The addressing of the device may be in two field times, one for switching to one stable state, and the other for switching into the second stable state. The field times may be identical or different in length. The device may be addressed by selectively switching pixels to one state in one field time and selectively switching pixels to the other state in the second field time. Alternatively, some or all of the pixels may be blanked into one state, then selectively switched to the other state. The blanking can be done at the same time to all pixels, a row at a time (e.g. one or more rows ahead of selective addressing), or the blanking and selective addressing may be combined as each row is being addressed.
The row waveform may be at least two unipolar pulses capable of blanking pixels, and at least two unipolar addressing pulses capable of combining with data waveforms to selectively switch pixels. The blanking pules may be of equal and opposite (or the same polarity) amplitude or different (including a zero) amplitude; similarly the addressing pulses may be of equal and opposite amplitude or different (including a zero) amplitude providing that overall the device receives substantially net zero dc voltage. The blanking pulses may be of the same or different amplitude to those of the addressing pulses. The two blanking pulses and the two addressing pulses may be equally or unequally spaced apart in time including blanking immediately followed by addressing. When the row waveform period is formed of three or more ts periods, then at least one time slot may be of zero voltage amplitude.
Each data waveform is usually of equal and opposite alternate pulses. However, for some applications a zero voltage may be applied in one time slot of each waveform period.
The row and data waveforms may have periods of two, three, four, or more time slots ts. The line address time may have periods of two, three, four, or more time slots ts. Furthermore, the row waveform period may extend in time over more than one line address time, in a manner analogous to the addressing of FELCDs in EP-0,542,804 PCT/GB91/01263.
The addressing may be to each row in turn, or in a different sequence, such as interleaving the addressing e.g. as in FIG. 11 below.
The temperature of the liquid crystal material may be measured and voltages Vs, Vd ratio of Vs/Vd and/or time length of ts, and/or relative position of blanking to selective addressing pulses adjusted to compensate for switching characteristics with temperature.
Additional voltage waveforms, voltage reduction waveforms, may be added to the row and or column electrodes. When added to row electrodes these voltage reduction waveforms combine with the column voltages without changing the required switching voltages to give an overall reduction in peak or rms. levels.
Use of voltage reduction waveforms gives reduced voltages requirements for driver circuits. This enables standard drivers circuits designed to rms. address twisted nematic type of displays, to be used as in GB 2,290,160.
According to this invention a bistable nematic device comprises;
two cell walls spaced apart and enclosing a layer of nematic or long pitch cholesteric liquid crystal material;
a first series of electrodes on one wall and a second series of electrodes on the other wall collectively forming a matrix of intersecting regions or pixels;
surface treatments on at least one wall to provide a molecular alignment permitting the molecules at or adjacent the wall to align into two different stable states upon application of appropriate unipolar voltage pulses;
means for distinguishing between the switched states of the liquid crystal material;
means for generating and applying a row waveform to each electrode in the first series of electrodes in a sequence;
means for generating and applying one of two data waveforms to each electrode in the second series of electrodes;
the row waveform having a period of at least two time slots and at least two unipolar pulses for switching the device to a first state, at least two unipolar pulses for switching the device to a second state;
both data waveforms having a period of at least two time slots with a unipolar pulse in each time slot giving substantially net zero dc value, with at least one data waveform shaped to combine with the row waveform to cause a switching to one latched state;
whereby each pixel can be independently switched into either stable state to collectively provide a desired display, with a substantially net zero dc voltage applied to the device.
The means for distinguishing between the switched states of the liquid crystal material may be two polarisers, or a dichroic dye in the liquid crystal material with or without one or more polarisers. The polarisers may be neutral or coloured.
The first series of electrodes may be formed into row or line electrodes, and the second series of electrodes formed into column electrodes. The row and column electrodes form collectively a x,y matrix of addressable pixels. Typically the electrodes are 200 xcexcm wide spaced 20 xcexcm apart. Other electrode configurations may be used. For example so called r-xcex8 arrangements. Also alpha numeric, or seven or eight bar arrangements may be made.
The surface treatment may be grating surfaces. The grating may be a profiled layer of a photopolymer formed by a photolithographic process e.g. M C Hutley, Diffraction Gratings (Academic Press, London. 1982) p 95-125; and F Horn, Physics World, 33 (March 1993). Alternatively, the grating may be formed by embossing; M T Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling; E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from a carrier layer.
The grating profile may be uniform over each complete pixel, or may vary within each pixel so that different voltage levels are needed to switch different areas of a pixel. For such an arrangement, more than two different data waveforms may be used.
The device may include driver circuits, logic arrays, inputs such as keyboards, or computer links to address the device. Alternatively, the device may be a cell only, with cell walls, electrodes, liquid crystal material, and surface alignment treatment. In the latter case, the device may include contacts for connecting to drivers etc. as required when changes are made to the display device. This utilises the bistable nature of the device. For example smart cards may display information that can be changed by external means such as driver circuits, radio, magnetic, or laser readers or addressers when inserted into control circuits etc.
Cells designed as smart cards may suffer from static effects when moved around, e.g. into pockets or wallets. To avoid possible static effects some or all of the electrodes may be connected together with resistive links. These allow a charge stabilisation at the electrodes to prevent unwanted changes in display. The links are of sufficient is value to allow the induced charges to equalise slowly without effecting the much higher frequency voltage changes occurring when the cell is addressed.
The device may include nematic material only, or nematic plus a small amount of a chiral or cholesteric additive such as cholesteric liquid crystal material, and may include an amount of a dichroic dye for enhancing observed colour.